1. Field of the Invention
The present invention relates to a projection lens system, and more particularly to a magnifying projection lens system for magnifying and projecting an image displayed on an electronic display tube onto a screen, and to a projection lens system suitable for magnified projection of a high precision image of a variable projection magnification, having a mechanism for correcting a variation of aberration created by focusing.
2. Related Background Art
In the past, the projection lens systems have been constructed with glass lenses, or plastic lenses having non-spherical surfaces in order to reduce size and weight. Those projection lens systems have applications in display apparatus for home use, audio-visual educational use, TV conference system or high precision image system. Recently, applications in high quality image display such as high precision image system have been rapidly increasing, and the prior art projection lens systems for those applications have several problems in their optical characteristics. Among others, a resolution, particularly a resolution in a periphery of an image is significantly insufficient.
As the projection lens system, it is required that the lens system is portable and can project an image from any point onto a screen, or can be mounted in an image receiver of varying projection distance. In most of the prior art projection lens systems, focusing is attained by varying one of the spaces between lenses of a lens system.
Those system may be used only when a variation of distance between the system and a screen on which an image should be projected is small but a performance of the system is significantly lowered for a variation of the distance over a large range. Accordingly, those lens systems are hardly put into practice. A related application has been filed as USSN 747,029 but it does not describe reduction of performance due to focusing.